Methanol synthesis process using a synthetic gas (mixed gas of CO and H2) as a main raw material (containing a small amount of CO2) is a basic process that is extremely important in chemical industry, and increase of efficiency of the process has been always desired in the past from the viewpoints of energy saving, economy, etc. One of the most important techniques in the methanol synthesis process is to provide a catalyst of high performance. As conventional catalysts, three-component catalysts, such as Cu/ZnO/Al2O3 catalyst (catalyst for industry at present, e.g., “Shokubai Koza (Lectures of Catalysts)”, vol. 7, edited by Catalysis Society of Japan, issued by Kodansha Ltd. on Jul. 20, 1989, pp. 21-39 (non-patent document 1)) and Cu/ZnO/SiO2 catalyst (JPA-1988-39287 (patent document 1)), are known.
On the other hand, methanol synthesis using CO2 and H2 as main raw materials has been particularly paid attention recently from the viewpoints of recycling of carbon resources and global environmental problem. In the synthesis of methanol from a raw material gas having a high content of CO2, a catalyst having higher activity than a catalyst adopted in the synthesis of methanol from the above synthetic gas has been desired because of thermodynamic equilibrium of the reaction and the reaction inhibition effect of water formed together with methanol (Applied Catalysis A: General, 38 (1996), pp. 311-318 (non-patent document 2)). In the synthesis of methanol from a raw material gas having a high content of CO2, lowering of catalytic activity presumed to be attributable to water formed together with methanol is very severe as compared with that in the synthesis of methanol from a synthetic gas. On that account, a catalyst having much higher durability than a catalyst adopted in the synthesis of methanol from a synthetic gas has been desired. The reason is that the three-component catalysts adopted in the above methanol synthesis are insufficient in the catalytic performance.
From such viewpoints, copper-based multi-component catalysts obtained by further adding components, such as copper/zinc oxide/aluminum oxide/zirconium oxide catalyst and copper/zinc oxide/aluminum oxide/zirconium oxide/gallium oxide catalyst, have been developed (see, e.g., JPA-1995-39755 (patent document 2), JPA-1994-312138 (patent document 3), Applied Catalysis A: General, 38 (1996), pp. 311-318 (non-patent document 2)). Moreover, a catalyst of high activity obtained by adding 0.3 to 0.9% by weight of colloidal silica or water-dissolved silica as silica and calcining the resulting product at 480 to 690° C. has been developed (JPA-1998-309466 (patent document 4)).
These catalysts have high activity, but the activity is not necessarily reproduced sufficiently in some cases even if they are prepared by the same formulation. It is generally known that stable pH in the precipitation step and sufficient washing of a precipitant are necessary in order to obtain a catalyst of high activity with good reproducibility (e.g., JPA-1977-76288 (patent document 5), JPA-1995-8799 (patent document 6), JPA-2007-83197 (patent document 7)). A large number of improving methods for that have been carried out, but it cannot be necessarily said that their reproducibility is good, and further improvement has been desired.
The above catalysts are useful because of high activity. However, the preparation steps include, for example, precipitation, aging, washing, filtration, drying, molding and calcining, and become long. In the industrial production, therefore, it is preferable to simplify the preparation steps, however little, to thereby reduce a burden in the production, and such improvement has been also desired.
It is known that the main cause of such lowering of catalytic activity of the copper-based catalyst is decrease of surface area due to sintering of copper. It is known that copper is a metal readily undergoing sintering among various metal catalysts, and it is necessary that the reduction temperature of a catalyst before the reaction and the reaction temperature should not be raised. For example, it is described in JPA-2004-298685 (patent document 8) that the reduction temperature of a Cu/ZnO catalyst is in the range of preferably 100 to 300° C., more preferably 150 to 250° C.; it is described in JPA-1994-254414 (patent document 9) that the reduction temperature of a Cu/ZnO/ZrO2 catalyst is in the range of preferably 100 to 300° C., more preferably 120 to 200° C.; and it is described in JPA-2007-83197 (patent document 7) that the reduction temperature of a Cu/ZnO/Al2O3 catalyst is in the range of preferably 100 to 300° C., more preferably 110 to 280° C., still more preferably 130 to 240° C. The reason is that if the reduction temperature is raised too high, the copper surface area is decreased by sintering of copper. Accordingly, improvement in the pretreatment method (reduction method) of a copper-based catalyst which does not cause sintering of copper has been also desired.